1. Field of the Invention
This invention relates generally to electronic filters, and relates more particularly to programmable finite impulse response (FIR) filters.
2. Description of the Relevant Art
A finite impulse response (FIR) filter is a type of electronic filter with a broad range of applications. Such filters are widely used in both digital signal procession and real-time digital video processing, and may be used in, for example, the public switched telephone networks, wide area networks, or local area networks, and may be used with either copper media or optical fiber.
Where electronic data is transmitted over copper unshielded twisted pair (UTP) cables, either high speed or low speed transmission may be used. Low speed data transmission over UTP cables is known as E1-T1 transmission. There are several standards for high speed data transmission over UTP cables, including the 802.X family of standards, the FDDI standard, and the SONET standard, which is also known as the ATM or SDH standard.
In such conventional high and low speed transmission over UTP cables, adaptive filtering is used to continuously analyze the individual data elements being transmitted and to continuously update the FIR filter. Such filtering uses adaptation algorithms to prevent errors in the data being transmitted, for example, from an echo signal from the transmitter. Such echo signals may be significantly stronger than the original signal, making detection of the received signal without error impossible.
The operation of such an adaptive filter and the associated adaptation algorithm involves two separate functions. The first is the filtering process designed to produce a specific output in response to a specific sequence of input data. The second is an adaptive process by which the parameters of the filter may be changed based upon an analysis of the inputs and outputs of the filter.
One type of FIR filter suitable for use in adaptive filtering is a transversal filter, or tapped delay line filter. As shown in FIG. 1, the output of such a filter is a weighted combination of voltages taken from uniformly spaced taps. The filter 10 contains a plurality of unit delay elements 12, multipliers 14, and adders 16. The filter is considered to be of the Mth order, where M-1 is the number of delay elements.
Each delay element introduces a delay of time t. When a delay element operates on an input U.sub.n, which is the filter input at an initial time t.sub.0 plus n*t, the resulting output is U.sub.n-1, i.e. the input one delay period before. Thus, U.sub.n-k is the filter input at a time t.sub.0 plus (n-k)*t, which is also the output of the kth delay element at time t.sub.0 plus n*t. Each multiplier multiplies the tap input to which it is connected by a filter coefficient referred to as the tap weight W.sub.k so that the multiplier connected to the kth tap input U.sub.n-k produces an output U.sub.n-k *W.sub.k.
The adders sum the outputs of the multipliers to produce the filter output. This overall output Y.sub.n is given by the formula ##EQU1## This weighted sample allows the filter to reduce noise in the input signal.
Various attempts have been made to simplify this type of filter. For example, some transversal filters have been based upon powers of two so that the signal can just be shifted one bit at a time instead of using multipliers.